This invention relates to a brake system for automotive vehicles with front wheel drive or rear wheel drive and, in particular, to such a system with brake slip and traction slip control. One such brake system includes an auxiliary-energy-assisted braking pressure generator, two hydraulically separated pressure medium circuits arranged so that one front wheel and one rear wheel are connected and so that the allocation is preferably diagonal, and a hydraulic auxiliary-pressure-supply system. Valve arrangements are inserted into the pressure medium paths leading from the braking pressure generator and from the auxiliary-pressure-supply system to the wheel brakes and from the wheel brakes to a pressure compensation reservoir. These valve arrangements hydraulically connect the braking pressure generator with the wheel brakes in one switching position and the auxiliary-pressure-supply system with the wheel brakes in a second switching position. Separating valves are also included which, in the traction slip control phase, lock the pressure medium connection toward the wheel brake of the non-driven vehicle wheels. Finally, wheel sensors and electronic circuits are included for the determination of the rotational behavior of the wheels and for the generation of electric braking pressure control signals by means of which the valve arrangements and the separating valves can be changed over.
There are already known lock-up-protected brake systems equipped with an auxiliary-pressure-supply system by means of which braking pressure is also metered into the driven vehicle wheels for the purpose of controlling the starting slip or rather the traction slip (German Published Patent Applications DE-OS Nos. 33 38 826 and 34 07 538). As compared with a mere lock-up control system which only controls the brake slip the extra expenditure in these known systems is relatively small because a large number of components required for lock-up control can also be used for the detection and braking of the wheels spinning in case of excessive traction energy. What is also required for starting slip control are the auxiliary-pressure-supply system, the sensors for detecting the rotational behavior of the wheels, the electronic circuits for processing the sensor signals and for generating the braking pressure control signals and some of the slip-controlling valves. The electronics, of course, must be expanded, but this can be done at relatively small expense. For the purpose of starting slip control, the known lock-up-protected brake systems also include a few additional valves by means of which braking pressure is fed from the auxiliary-pressure-supply system into the wheel brakes of the driven wheels without actuation of the braking pressure generator.
There has already been suggested a brake-slip-and-traction-slip-controlled dual-circuit brake system of the type mentioned above for vehicles with diagonal brake circuit allocation (German Published Patent Application No. P 35 27 190). Inserted into the pressure medium paths from the inlet valves to the wheel brakes of the non-driven wheels of this known system are separating valves operative to ensure that only the driven wheels are braked in the traction slip control phase after the switching-off of the auxiliary-pressure-supply system. In this known system it is not possible to connect any non-return valves opening toward the braking pressure generator in parallel with the inlet valves connecting the braking pressure generator with the wheel brakes as in the traction slip control phase in which the brake pedal is not operated. Thus, the master cylinder or rather the braking pressure generator remain unpressurized and said non-return valves would prevent a pressure build-up in the wheel brakes of the driven wheels. However, for lock-up control and for safety reasons such non-return valves are desirable.